The centimeter-long "biobot" was made by attaching heart muscle cells onto a flexible structure, or body, of hydrogel—the same material used to make contact lenses for human eyes.

Watch a Video of the Biobot

To make the biobot's body, the team used a 3-D printer, which creates solid objects by laying down successive layers of soft materials that fuse together and harden.

Gathering the heart cells was a bit trickier. The researchers removed whole hearts from anesthetized newborn rats, cut the organs into tiny pieces, and then processed the fragments to loosen and separate the heart cells. The cells were then added to the robot body—each bot contains between a few thousand and a few hundred thousand. (Read "Heart Cells Can Regenerate, Nuclear-Bomb Evidence Shows.")

"In a few days they start beating, and the bots start to move," explained study co-author Rashid Bashir, an engineer at the University of Illinois at Urbana-Champaign who helped develop the robot.

As the biobot's "engine," the heart cells' contractions bend the machine's body, causing it to move forward fractions of an inch per second. The biobot has two legs, one that propels it forward and another that acts as a stabilizer.

Heart cells were chosen for the biobot because they spontaneously contract, or "beat," in time with one another, Bashir said by email.

For now, the biobot must be submerged in a nutrient-rich fluid to keep the heart cells alive. But future biological machines could be "fed" via veins.

"Work going on in other labs is aimed at creating vascular systems to meet the metabolic needs of muscles for biobots as they become more developed and grow in scale," said Roger Kamm, a mechanical engineer at MIT who was not involved in the study.

By melding the synthetic and the natural, engineers hope to endow their creations with biological abilities that purely mechanical robots just aren't capable of yet.

"There's a lot that biology does that we just haven't been able to replicate with the inanimate materials that we currently have," Kamm said. "For example, the nose is just a fantastic sensor. We still use dogs in airports to sniff out explosives."

Biobot researchers say there is no limit to the potential uses for their creations.

"You could have crawling or swimming biobots that could sense and migrate towards—and then neutralize—toxic substances," Kamm said.

Similarly, he added, "you could also imagine biobots that function inside the human body and that could sense [chemicals] secreted by tumor cells, migrate through the tissue to the tumor, and secrete substances that destroy it."

But all of that is still far off in the future, study author Bashir cautioned.

"For now we are working on understanding the underlying principles and design rules."